This invention relates to a combustion device to be used in oxidation devices for oxidizing objects to be treated, such as semiconductor wafers.
The fabrication of semiconductor devices includes the oxidation step for forming oxide films on the surfaces of semiconductor wafers. In one technique for this oxidation, semiconductor wafers are contacted with water vapor at a high temperature in an oxidation furnace to form oxide films on their surfaces. The oxidation device for this technique using water vapor is known in, e.g., Japanese Patent Laid-Open Publication (KOKAI) No. 623326/1981, in which hydrogen gas and oxygen gas are fed directly in an oxidation furnace to combust in the oxidation furnace, and water vapor is generated.
But in this oxidation device, in which hydrogen gas and oxygen gas are combusted in the oxidation furnace, the temperature in the oxidation furnace greatly depends on combustion ratios between the hydrogen gas and the oxygen gas. Accordingly it is difficult to sufficiently control the temperature for oxidizing semiconductor wafers. The reliability, safety and reproducibility of the oxidation of semiconductor wafers are low.
In another oxidation device known in, e.g., Japanese Patent Publication (KOKOKU) No. 60528/1988 and Japanese Patent Laid-Open Publication (KOKAI) No. 210501/1988, an external combustion device is provided as a water vapor generating device separately outside an oxidation furnace, and this external combustion device is fed with hydrogen gas and oxygen gas and combusts the same to generate water vapor, which is supplied to the oxidation furnace through a pipe.
This external combustion device allows the heating state in the oxidation furnace to be controlled independently of the operational state of the external combustion device as the water vapor generating device. Accordingly the oxidation of semiconductor wafers in the oxidation furnace can be conducted with high reliability, safety and reproducibility. This external combustion-type oxidation device has been recently noted.
On the other hand, recently semiconductor wafers, which are subjected to the oxidation, have increased diameters, as of 6 inches or 8 inches. To efficiently oxidize semiconductor wafers of such large diameters, it is necessary to feed large amounts of water vapor to oxidation furnaces. Also to this end, it is necessary to generate large amounts of water vapor by using external combustion devices as water vapor generating devices.
For the generation of a large amount water vapor by an external combustion device, large amounts of oxygen and hydrogen have to be fed into and combusted in a combustion vessel of the external combustion device. This results in increases of the gas combustion temperature in the combustion vessel, which cause various troubles. Especially the supply amount of the hydrogen gas has been restricted.
To be specific, conventionally in FIG. 8, large amounts of hydrogen gas and oxygen gas are injected for combustion, heated up to their spontaneous ignition temperatures, from the adjacent end nozzles 50', 51' of a hydrogen gas feed pipe 32 and an oxygen feed pipe 33 which are in a double pipe. Flame is generated from the position of the forward end 32a of the hydrogen gas nozzle at the center of the double pipe. The flame heats the forward end 32a up to a temperature above its devitrification temperature (about 1200.degree. C.) and devitrifies the same (phenomenon that transparent quartz is made opaque to peel off in flakes). This devitrification results in generation of particles which lower yields of products, and deformation of configurations of the nozzles and clogging of the nozzles. In addition, the flame elongates in shape to reach the inside upper wall of the combustion vessel, which is adversely heated to abnormally high temperatures. Thus, although it is preferable to increase the hydrogen combustion amount so as to increase the water vapor generation amount, it too raises the temperature. In addition, the gas feed amount is restricted, and the capacity of the combustion vessel itself has to be increased. These will lead to the problems of higher costs of the combustion device, and larger spaces for the installation of the device.